1. Field of the Invention
The present invention relates to an image forming apparatus, and particularly relates to an image forming apparatus that forms an image by means of toner.
2. Description of Related Art
As a conventional image forming apparatus, there is known, for example, an image forming apparatus described in Japanese Patent Laid-Open Publication No. H08-272235. Hereinafter, the image forming apparatus described in Japanese Patent Laid-Open Publication No. H08-272235 is described with reference to the drawings. FIG. 8 is a constitutional view of an image forming apparatus 500 described in Japanese Patent Laid-Open Publication No. H08-272235.
The image forming apparatus 500 includes an image carrier 502, a transfer member 504, and a bias application device 506. The image carrier 502 electrostatically carries a toner image. The transfer member 504 is in contact with the image carrier 502, and applies a transfer bias. The bias application device 506 sequentially applies to the transfer member 504 bias currents with different polarities from each other while the transfer member is not present at a transferred position. The bias application device 506 then applies a same-polarity current having the same polarity as toner constituting the toner image, and thereafter applies a reverse-polarity current having a reverse polarity to the toner and a current value equal to or greater than an absolute value of the same-polarity current. It is thereby possible to transfer toner from the transfer member 504 back to the image carrier 502 for cleaning, so as to prevent staining on the back of recording paper.
However, in the image forming apparatus 500 described in Japanese Patent Laid-Open Publication No. H08-272235, the toner adhering to the transfer member 504 cannot be sufficiently removed. More specifically, the bias application device 506 applies the same-polarity current having the same polarity as the toner constituting the toner image, and thereafter applies the reverse-polarity current having the reverse polarity to the toner and the current value equal to or greater than the absolute value of the same-polarity current. Hence in the case of the same-polarity current being small, the toner remains on the transfer member 504 after application of the same-polarity current. In this state, when the reverse polarity current having a larger absolute value than that of the same-polarity current is applied, the toner is drawn to the transfer member 504, and a large amount of toner remains on the transfer member 504.
On the other hand, in the case of the same-polarity current being large, discharge occurs due to the same-polarity current. When air in the vicinity of the transfer member 504 is decomposed by the discharge, a same-polarity ion with the same polarity as the toner and a reverse-polarity ion with a reverse polarity to the toner are generated. Then, the reverse-polarity ion is drawn to the transfer member 504 by the same-polarity current, and the polarity of the toner is reversed by the reverse-polarity ion. Subsequently, when the reverse polarity current having a larger absolute value than that of the same-polarity current is applied, the polarity of the toner, which was once reversed, is reversed again due to discharge. As a consequence, the toner is drawn to the transfer member 504 by the reverse-polarity current, and remains thereon. As thus described, in the image forming apparatus 500 described in Japanese Patent Laid-Open Publication No. H08-272235, it is difficult to sufficiently remove toner from the transfer member 504.